This invention relates to a novel method for the combined drying, curing, and mining of bentonite clay.
Bentonite clay is found in beds of varying thickness. Overburden depth usually dictates that the economically mineable thickness of the bentonite beds be from two to about four feet or thicker. The means or techniques by which these beds are harvested and processed determine the clay's eventual potential and/or value as an industrial mineral.
Bentonite clay is made up of extremely small plate-like crystals stacked in layers. Due to inherent crystallography and certain characteristic charge densities thereon, bentonite possesses the ability to organize and rigidify water upon and near the faces of the individual crystals. In situ bentonites hold organized water at levels of thirty-two percent. Bentonite as such is of no real economic or industrial value. Bentonite must be processed and dried to certain critical levels of moisture content of about 12% and the techniques involved in so processing and drying are of the greatest importance if the ultimate potential and value of the bentonite is to be realized. Bentonite can be, and is, overdried. Overdrying is brought about by removal of all the water from between the faces of the clay platelets with the concomitant face to face junction of the crystals. Certain forces (Van der wals) then make the readsorption of water, for all practical purposes, impossible, thus destroying much, if not all, the bentonite's ultimate value. Underdrying, though less destructive than overdrying, provides a bentonite clay of sluggish and inadequate response.
Bentonite clay is generally mined in large lumps containing large amounts of moisture. It is, therefore, necessary to dry and reduce the size of the clay before it is sold in order that it may be more easily transported and used in subsequent industrial processes. Ladoo and Meyers, Non-Metallic Minerals, Second Edition (1951), pages 96-98.
Early methods for mining bentonite consisted of digging by hand or plowing the bentonite with a team of horses than raking the clay to dry over steel plates heated by coal or wood fires (Wolfbauer, C. A., Bentonite in the Powder River Basin, Wyoming Geological Association 28th Annual Field Conference Guidebook, 1976 Geology and Energy Resources of the Powder River, Casper, Wyoming, September 1976, Ed. Landon, R. B.). Bentonite mined by these early techniques was found in outcroppings which were already dried and cured to a large extent, because of exposure to the atmosphere. In principle, the early methods of mining bentonite and the conventional methods used today are the same. The wet clay is removed from the pit and transported to a drying facility. In the early methods, the drying facility was a heated steel plate located in the pit whereas today the drying facility is a modern plant located many miles from the pit.
In the conventional mining of bentonite clays the overburden is removed to expose the bentonite. The bentonite clay displays the natural ability to block the passage of water. It is then usual that the exposed bentonite within a pit be drained peripherally, such that no water be allowed to collect or build up thereon. This process is commonly referred to in the art as "cupping" the bentonite clay pit. The efficient and prudent use of bentonite as presently mined dictates the clay be cured an extended period of time prior to plant processing. The term curing as used herein refers to that natural process which is evidenced as bentonite dewaters through exposure to the natural drying effects of the environment. Bentonite when so exposed delaminates and exfoliates from the surface of the ore body to a given but shallow depth. This delamination and exfoliation is characterized by a granule which attains dimensions nearly spherical and which is driest at its core; for as the uppermost surface dries it shrinks, thereby lifting a shoulder of wetter clay up and around the dryer portion until the process is culminated by the forming granule plucking itself free from the wetter ore body. This low density granule will attain a dimension roughly one-half inch in diameter and has been referred to within the industry and the literature as a "popcorn" structure. The total exposed surface of an ore body, outcrop or pile will display this phenomenon quite swiftly. Due to the protective nature of this dried-cured bentonite, that portion of the ore body which underlies the cured bentonite will remain at or near its natural state.
As a naturally-occurring process, an ore body will cure to a thickness of from one-half to one inch in depth. It has been found if the uppermost surface of the ore body is broken up curing will progress at a swifter rate more deeply into the ore body. Depending upon prevailing environmental conditions, the cured layer will possess from ten to about eighteen percent moisture.
Conventional clay mining techniques involve removal of the entire thickness of the clay bed at one time using equipment such as endloaders. If time permits, the surface of the clay deposit may be tilled and exposed to atmospheric drying for a period of from six to eighteen months; however, even if this technique is used, the clay only cures to a depth of a few inches to a foot, whereas, the vast majority of the deposit remains in an uncured state.
Original ore moisture is about thirty-two percent. Clay mined by conventional techniques generally contains moistures in the range of from twenty-five percent to thirty-two percent. Very little drying or curing is evidenced prior to harvesting, even when the surface of the clay deposit is tilled prior to removal. That small amount of clay which enjoyed some drying and curing is harvested along with that vastly larger bulk of clay which is undried and uncured. The entire clay deposit is placed into trucks which traverse long distances, hauling a material which contains from twenty-five percent to thirty-two percent moisture. Common practice among clay mining companies is to have the wet clay hauled to the processing plant by contract haulers, whose charges are based on the weight of the clay. Thus, producers pay considerable sums for the hauling of excess moisture in the clay. Furthermore, the wet clay as mined from the earth is sticky and difficult to handle. It sticks to the buckets of endloaders and to truck beds, often requiring operators to slam the buckets against the truck bed to dislodge the clay. This technique is time consuming and, of course, causes severe wear and tear on the equipment.
The clay is conventionally dumped on stockpiles adjacent the processing plants. Heavy machinery is used to spread the dumped clay over large wedge-shaped stockpiles. While conveying and spreading the clay on the piles, the machinery compresses and densifies the stockpiles. Such compacted clay seldom falls below a moisture level of twenty-five percent and is of variegated sizing, from near dust to compacted lumps in excess of one foot in diameter. Due to the sizing and the moisture level of the clays removed from the stockpiles, it is necessary that the clay pass through a special crusher which is externally heated to preclude sticking and clay buildup within the crusher. The special externally heated crusher is a necessary step prior to the finish drying of clay as it is presently mined. Clay is circulated through the heated crusher until it is of a size most efficiently managed in the drying process. Bentonite clay mining, using present techniques, is finish dried to a moisture level of about 9% from a level of from twenty-five percent to thirty-two percent moisture.
As clay is presently mined and processed there is, of necessity, a compromise struck in the drying process. Within the finish dryer exists granules of clay in excess of one-inch diameter with grading down to dust sized particles. The small percentage of dust sized particles are usually blown out or through a dryer swiftly. The larger granules and chunks are overdried peripherally and underdried at the core for bentonite effectively blocks the movement of water from the core to the periphery. As the outer surface of the granule or chunk dries, it heals over or armors itself against the ingress or egress of water. At finish grinding there then exists a bentonite product made up of a compromise average of overdried and underdried clay mixed with a small percentage of adequately dried clay. The most important factors influencing this type of inefficient, wasteful and destructive drying is the high moisture content and uncured state of the incoming bentonite clay.
It is apparent that large amounts of energy are required to transport, crush, and dry the wet clay to a granular material having a moisture content of about 9%. In the interest of a more intelligent, judicious, efficient and ultimately more conservative utilization of the natural resource, bentonite, and a more efficient and conservative usage of energy, man made and natural, the invention of a novel mining, drying and curing technique for the processing and manufacture of bentonite is herein described.